Refractory period

Bioelectricity: Electric excitability and action potential

Refractory period

In the following tasks, it may be handy to use the simulation of the Hodgkin-Huxley model in widescreen format. This use interface has been simplified in the sense that Nernst potentials cannot be changed.

Tasks

Set the second stimulus also at a magnitude of 10 µA/cm² and run the simulation. What happens?
Set the second stimulus again at a magnitude of 10 µA/cm², but now already started after 10 ms, and start the simulation. What happens?
Set the starting time of the second stimulus again at 10 ms, change the current magnitude to 50 µA/cm². What happens?

Answers

When you inject current twice and allow for sufficient time between the stimuli, then you get two action potential generated.
When there is such a short time between the two stimuli that the sodium channels are still inactivated after the first stimulus when the next one happen, then no second action potential is generated
Also with a second stronger stimulus shortly after the first stimulus, no action potential is generated; the cell is not able to fire. An absolute refractory period exists in which the cell cannot fire again unless given an unrealistic physiological stimulus (i.e., a stimulus with an extremely high intensity).